PROJECT SUMMARY/ABSTRACT Meiosis in sexually reproducing organisms is critical for generating genetic diversity and the production of healthy gametes. The synaptonemal complex is a dynamic and conserved macromolecular protein complex which is crucial for chromosome pairing, meiotic recombination and proper chromosome segregation. Errors or defects in these processes result in zygotes, which die early in development, infertility and aneuploidy, resulting in developmental disorders like Turner syndrome, Down syndrome. The lack of structural framework for the synaptonemal complex has hindered mechanistic insights into its role in critical meiotic processes. My proposal combines innovative electron tomography methods and molecular biology tools to define the complete molecular architecture of the synaptonemal complex in paired chromosomes using the tractable systems, C. elegans and S. cerevisiae. During the K99/R00 period I will, 1) Construct the spatiotemporal landscape of synaptonemal complex architecture changes from early to late pachytene in C. elegans for all six chromosome pairs and highlight the role played by key protein factors in regulating the concerted change in synaptonemal complex architecture and meiotic recombination processes. 2) Determine a high-resolution molecular view of the synaptonemal complex for both S. cerevisiae and C. elegans in order to (a) provide insight into its subunit organization and evolutionary conservation, (b) examine its interactions with both chromosome axis proteins and recombination nodules - crossover machinery, to delineate its role in homologous recombination and chromosome architecture changes during pachytene. 3) Provide mechanistic insight into homologous chromosome recognition by elucidating the role played by nuclear envelope proteins in regulating cytoskeletal forces (microtubules, dynein) and chromosome movements critical for homolog recognition and subsequent initiation of chromosome pairing. During my postdoctoral work in the Nogales lab, I have obtained training in single-particle cryo-electron microscopy and provided structural insights into the regulation of chromatin modifier, Polycomb Repressive Complex 2. During my K99/R00 phase, I will undertake training in electron tomography methods and genetics tools that will enable me to study critical processes during meiosis, in vivo. I am confident that my training in electron tomography coupled with the excellent mentorship of Eva Nogales, Elizabeth Villa, Abby Dernburg, and the rest of my advisory team, will help me transition to an independent research career. I believe my access to top notch scientific infrastructure and a truly collaborative scientific community at UC Berkeley makes it the ideal environment for my K99/R00 training. During my R00 phase, I will provide insight into the molecular mechanisms governing homologous chromosome recognition and how defects in these mechanisms lead to chromosomal disorders. I envisage developing a cross-disciplinary research group utilizing electron microscopy, biochemistry, and computational tools to address problems concerning chromosome architecture and transcription regulation.